Electronic Design

IGT Creates A Low-Cost Off-Line "Hiccup" Supply

In equipment requiring a standby mode (e.g., VCRs, TVs), you must still supply power to the microprocessor when everything is asleep in order to receive and interpret the wake-up signal from the remote control. The power consumption of the microprocessor in sleep mode is rather low, and classical switchedmode power-supply ICs are clearly overkill at the 100-mW to 200-mW output levels required. The active solution also must be cost effective compared to the standard topology using a metallic transformer.

The schematic shows how a UC3845 switched-mode IC can be used to drive the MMG05N60D, a 600-V IGBT recently introduced by Motorola (Fig. 1).

For this application, the typical voltage-feedback functionality of the 3845’s PWM output generator is forced to its maximum setting by grounding the internal error amplifier via the feedback pin. In this configuration, only the clock generator and the peak current detector will remain active.

When power is applied, resistor R3 pulls the C4 capacitor’s potential above the 3845’s under-voltage lockout level (UVLO). The startup current doesn’t exceed a few hundred microamperes. Consequently, power isn’t wasted. When IC1 begins operation, pin 6 delivers a driving signal to the IGBT (M1). Thanks to its insignificant gate charge (5 nC), the IGBT doesn’t excessively load down the C4 voltage and a burst of pulses can take place. Power is transferred to the output through XFMR1’s secondary winding when M1 turns off as in classical flyback mode.

The UC3845’s operating current discharges C4 until the supply falls under the UVLO level. IC1 then stops operating and enters startup mode again. A “hiccup” operation, which has a recurrence defined by IC1’s operating current (R5 and C4), then takes place. The peak current is regulated by the UC3845, allowing operation over universal mains supplies. Because the circuit operates at constant output power, the peak current can be extracted by the following formula:

With an internal error amplifier clipped at 1 V, R2 is simply 1/Iρ.

Thanks to its avalanche capability, the MMG05N60D doesn’t require any clipping network, as Figure 2 demonstrates by magnifying a burst, using a 250-V ac input line. The IGBT case temperature was less than 40°C at an ambient of 25°C.

The transformer XFMR1 is can be an off-the-shelf model as long as the supply operates in discontinuous mode, as with the component values given in Figure 1.

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